Automotive vehicles driven by liquid cooled engines generally employ a passenger compartment heating system having a flow tube type heat exchanger (heater core) through which engine coolant flows. Air passing over the heater core is heated and directed into the passenger compartment to warm the compartment. The coolant flow rate through the heater core affects the amount of heat transferred into the passenger compartment. Coolant control valves have been employed for governing the coolant flow through the heater cores to control passenger compartment temperatures.
Liguid cooled automotive vehicle engines generally employ a coolant composed of at least 50% ethylene glycol, or an equivalent compound, and water. Ethylene glycol prevents the coolant from freezing in cold climates, or seasons, and is usually thought of as an "antifreeze" additive; however, ethylene glycol is an extremely effective wetting agent. As such, ethylene glycol greatly increases the ability of engine coolant to gain or lose heat while flowing through engine cooling passages or heat exchanger tubes, even though the heat capacity of ethylene glycol is not as great as that of water. Thus the presence of ethylene glycol in engine cooling systems is desirable regardless of ambient atmospheric temperatures.
Engine coolant systems typically contain abrasive particulate matter which circulates with the coolant. The particulates, together with ethylene glycol, tend to foul coolant flow control valves. Fouled valves can stick, or jam, in a particular position and fail to control the coolant flow in response to passenger compartment temperature. This nearly invariably results in vehicle occupant discomfort and necessitates control valve repair or replacement.
Engine coolant flow control valves have been proposed in which variable proportions of the coolant flowing to the control valve bypasses the heater core. Examples of such valves are disclosed by U.S. Pat. Nos. 3,477,498 and 4,195,777. Valves of the sort disclosed by these patents are of relatively complex construction in that they contain many component parts including multiple valving members and are not easily assembled. Such valves can be relatively expensive to purchase, particularly as replacement parts, and are sometimes subject to being fouled and rendered inoperative by materials in the engine coolant flowing through them. In addition the pressure drop of the coolant flowing through the valve can change substantially between the heating condition and the bypass condition.